Effects of Aluminum and Zirconia Contents on the Sintering of Reaction-Bonded Aluminum Oxide Ceramics

The reaction-bonded aluminum oxide (RBAO) process is an attractive alternative to the conventional processing of Al₂O₃-based ceramics. The most attractive features of the process are the high strengths, densities, and easy machinability of the green powder compacts, and the low shrinkage and high strengths of the sintered ceramics. These advantages result from the presence of aluminum in the green bodies and are enhanced further with increasing aluminum contents. However, it is apparent that ZrO₂-containing RBAO powders with higher aluminum contents (>45 vol%) are increasingly more difficult to densify, as the start of densification is delayed (shifted to higher temperatures) and the densification rates are decreased. Ultimately, this results in a decrease in the limiting density to which the RBAO ceramic may be sintered. In this study, the cooperative effects of ZrO₂ and aluminum contents on the sintering of RBAO ceramics are discussed in terms of densification behavior and microstructural analysis.     

Conventional and Microwave-Induced Sintering Mechanisms for Reaction-Bonded Aluminum Oxide with Zirconium Oxide Additions

Powder compacts consisting of Al, Al₂O₃, and ZrO₂ were heated by microwave radiation. Tracing the phase evolution during reaction bonding revealed the reaction mechanism. In the case of conventional heating, the compacts expanded slightly at temperatures of <700°C due to Al surface oxidation and expanded sharply at temperatures greater than 700°C as oxidation proceeded from the surface to the interior. Then, the compacts shrank at 1550°C due to sintering. For the case of microwave heating, the compacts expanded at temperatures of <550°C due to the formation of Al₃Zr. This Al₃Zr formation was caused by the preferential heating of ZrO₂ relative to Al and Al₂O₃ by microwave radiation. Then, Al₃Zr was oxidized to form Al₂O₃ and ZrO₂ at temperatures of >1000°C. Finally, the compacts shrank at 1550°C due to sintering, similarly to conventional sintering.     

Feedback-Controlled Firing of Reaction-Bonded Aluminum Oxide

The reaction-bonded aluminum oxide (RBAO) process utilizes the oxidation of intensely milled aluminum/alumina powder compacts that are heat treated in air to make alumina-based ceramics. RBAO samples are typically oxidized in a furnace which is heated at 1^°C/min to 1100^°C. Heat-treating samples with a characteristic dimension >1 mm, without adjusting the furnace temperature program, usually results in a cracked ceramic. Cracking is caused by the excessive thermal and chemical stresses that result from steep temperature gradients (>30^°C/mm) and compositional gradients (>5000 mol·(m³·mm)⁻¹), which develop under the deleterious ignition and shrinking core reaction regimes. While adjustments to the furnace temperature program based on continuum models have had some success, the use of feedback-controlled firing is investigated as a means to avoid the furnace temperature program design step and to decrease the firing time. Feedback-controlled firing is shown to improve yields and significantly reduce the time required to completely oxidize the aluminum. For example, a 16 g sample with a characteristic dimension of 7.56 mm, which previously took >100 h to oxidize completely, was successfully oxidized crack free in 18.3 h using feedback control. Using the typical heat-treatment cycle, a 1 mm sample was fired in 18 h. With feedback-controlled firing, the same sized sample was fired in only 5 h.     

Seeding of the Reaction-Bonded Aluminum Oxide Process

The effect of the initial α-Al₂O₃ particle size in the reaction-bonded aluminum oxide (RBAO) process on the phase transformation of aluminum-derived γ-Al₂O₃ to α-Al₂O₃, and subsequently densification, was investigated. It has been demonstrated that if the initial α-Al₂O₃ particles are fine (∼0.2 μm, i.e., 2.9 × 10¹⁴γ-Al₂O₃ particles/cm³), then they seed the phase transformation. The fine α-Al₂O₃ decreases the transformation temperature to ∼962°C and results in a finer microstructure. The smaller particle size of the seeded RBAO decreases the sintering temperature to as low as ∼1135°C. The results confirm that seeding can be utilized to improve phase transformations and densification and subsequently to tailor final microstructures in RBAO-derived ceramics.     

Controlled Firing of Reaction-Bonded Aluminum Oxide (RBAO) Ceramics: Part II, Experimental Results

The reaction-bonded aluminum oxide (RBAO) process is an attractive alternative to conventional processing of ceramics, because of advantages such as lower costs, enhanced green machinability, near-net-shape forming, and broad microstructural variability. However, various problems are still encountered in the production of RBAO ceramics. Part I of the paper presented model predictions that may allow for the controlled firing of RBAO ceramics. The current work investigates the reaction behavior of RBAO ceramics under the model-predicted conditions (i.e., for varying oxygen content, heat loss, heating cycles, and scale) via thermogravimetry, differential thermal analysis, and analysis of samples that have been fired in a box furnace. By controlling the reaction, one can produce large, crack-free RBAO ceramics.     

Controlled Firing of Reaction-Bonded Aluminum Oxide (RBAO) Ceramics: Part I, Continuum-Model Predictions

The reaction-bonded aluminum oxide (RBAO) process is a novel, reaction-forming technique for producing monolithic, alumina-based ceramics. Although there has been extensive work on the RBAO process, it is often difficult to reproduce the process and avoid sample cracking. To solve the problems that are associated with the RBAO process, it is necessary to have a fundamental understanding of the reaction-bonding process and the effects of various processing parameters on the reaction behavior. To gain some insight into the process, a continuum model has been developed. The model, which considers the interaction between the macroscopic material and energy balances, is used to predict conditions under which RBAO bodies may be fired in a controlled manner, i.e., avoiding the runaway reaction. In particular, the effects of the oxygen content of the atmosphere, the heat loss by convection and radiation, the heating cycle, and scale (sample size) have been investigated. For small sample sizes, model predictions indicate that the reaction may be controlled by reducing the oxygen content of the atmosphere, increasing the heat loss, and/or incorporating an isothermal hold into the heating cycle at a temperature just below the ignition temperature. For larger sample sizes, model predictions indicate the need for multiple low-temperature holds at increasing temperatures. It is believed that firing RBAO bodies in a controlled manner will allow one to avoid sample cracking. Part II of this work presents a complementary experimental study that investigates the reaction behavior and structural integrity of samples that have been fired under the predicted conditions.     

Effects of Milling Liquid on the Reaction-Bonded Aluminum Oxide Process

The reaction-bonded aluminum oxide process begins with aluminum, Al₂O₃, and usually ZrO₂ powders that have been attrition-milled in an organic liquid. The attrition-milled powder is then compacted and heat-treated in air to produce polycrystalline, Al₂O₃-based ceramics. Safety considerations have made it desirable for the milling liquid to be changed from acetone to a less-flammable solvent. In this paper, mineral spirits, ethanol, and mineral spirits that contains 2 wt% stearic acid are presented as viable alternatives to acetone. The effects of changing the milling liquid on the reaction process and the properties of the final fired ceramic are investigated.     

Kinetics and Mechanisms of High-Temperature Creep in Silicon Carbide: I, Reaction-Bonded

The kinetic characteristics and the controlling mechanism of steady-state creep were determined for NC–430 reaction-bonded silicon carbide which was subjected to high temperatures (1848 to 1923 K) and constant compressive stresses (110 to 220 MN/m²). Both as-received and as-crept materials were studied extensively by transmission electron'microscopy as one means of determining the controlling creep mechanism. Small variations in sample density resulted in large variations in the creep rate. The stress exponent, n in the relation εασⁿ, was found to be 5.7 and the creep activation energy 711 ± 20 kJ/mol. The controlling creep mechanism was determined to be dislocation glide/climb controlled by climb.     

Composite Aluminum Oxide Ceramics with Fibrous Components

The reactions between oxides in the Al₂O₃ – ZrO₂ – MgO system in the production of ceramics using fibrous components are studied. It is established that under heat treatment of the ternary systems, the component stabilizing the tetragonal structure of zirconium dioxide reacts with the aluminum oxide matrix and forms spinel interlayers on the fiber – matrix interface. The use of highly disperse fibers as the initial component for producing ceramics and as a fibrous filler additive introduced into a gel-like matrix makes it possible to obtain composite ceramics of elevated strength.     

Porous Permeable Ceramics Based on Aluminum Oxide

The effect of additives (within limits of 15 – 30%) of disperse alumina with different content (up to 2.3%) of TiO₂ and MgO on properties of porous permeable materials based on electromelted corundum of different degrees of dispersion is investigated. Porous ceramics with open porosity up to 40% and bending strength up to 25 MPa is obtained, which has high filtration efficiency and chemical stability.     

Water Content and Water Evolution from Reaction-Bonded Aluminum Oxide (RBAO) Powder Precursors

Water evolution behavior from reaction-bonded aluminum oxide (RBAO) powder precursor was studied by the Carl-Fischer titration method. RBAO mixtures were milled for 4 and 8 h in absolute ethanol and water–ethanol mixtures containing 5 and 10 vol% water. The water content was then determined under dynamic heating conditions from 20°C to 250°C and static heating at 250°C. When the aluminum content is increased from 30 to 60 vol%, the water content of the milled powder precursor mixtures also increased. The higher water contents observed in powders milled in absolute ethanol are discussed in terms of the possible interaction of the ethanol with the surface of the aluminum particles.     

Electrical Properties of Laser-Synthesized Aluminum Oxide-Tungsten Oxide Ceramics

Electronic ceramics of the Al₂O₃-WO₃ system were synthesized by using a high-powered CO₂ laser. It was shown that these ceramics can be used as linear thermistor materials. The linear temperature range of the Al₂O₃-WO₃ system was ~200°C, the nonlinear deviation epsilon was <2%, the thermistor constant B was <10³, and the temperature coefficient was between -0.2%/°C and -0.8%/°C. These parameters could be adjusted by changing the composition of the samples and the laser-synthesis parameters. The present study showed that the materials should be encapsulated if they are to be used in an oxidizing atmosphere. The current-voltage curves of the present materials were symmetrical around the coordinate origin, which also is a necessary condition for judging whether or not a thermistor is linear.     

Processing and Properties of ZrO2-Containing Reaction-Bonded Aluminum Oxide with High Initial Aluminum Contents

The reaction-bonded aluminum oxide (RBAO) process relies upon the oxidation of Al/Al₂O₃ powder compacts, and many of its associated advantages stem from the presence of the aluminum in the green powder. Higher aluminum contents in the starting powders allow for higher green strengths, densities, and lower overall shrinkage, all while producing a fine-grained, high-strength sintered material. However, it is evident that the reaction and sintering of ZrO₂-containing RBAO with higher aluminum contents are more challenging. Therefore, in this study, the effects of aluminum content on the processing, structure, and properties of RBAO ceramics were comprehensively characterized. It was found that RBAO samples with high aluminum contents were more prone to cracking during reaction and even when successfully fired were not able to be sintered to full density. Despite these characteristics, RBAO samples with increasing aluminum contents showed no significant degradation in mechanical properties.     

Kinetics and Mechanism of the Reaction Between Zinc Oxide and Aluminum Oxide

Quantitative X-ray diffraction methods were used to define the kinetics of spinel formation. Diffusion controlled reaction rate constants were calculated on the basis of the reaction models of Jander, Dunwald-Wagner, Valensi, Zhuravlev et al. , Ginstling-Brounshte'n, and Kroger-Ziegler. The model proposed by Valensi is valid for describing solid-solid reaction rates in the later stages of the reaction. Inert markers indicated that diffusion of zinc ions through the spinel layer is the rate controlling mechanism. The activation energy for the process is 54,200 cal/mole. In the early stages of the reaction there is a second-order phase boundary kinetic process with an activation energy of 28,700 cal/ mole. The rate of chemical combination at the zinc-oxide-spinel phase boundary is believed to be rate controlling.     

氧化铈和氧化镁复合改性钛酸铝复相陶瓷性能的研究

钛酸铝是目前所知唯一集低膨胀和耐高温于一体的结构材料.但它在850～1280 ℃时会分解成氧化铝和氧化钛,并且其强度也很低,这两个弱点严重制约了它的使用范围.以CeO2和MgO为添加剂,对钛酸铝陶瓷进行复相改性.用万能材料试验机和扫描电镜等研究了材料的体积密度、热膨胀系数、机械性能和显微结构等.结果表明CeO2和MgO可以有效地改善钛酸铝复相陶瓷的各种性质,且添加(4～6)%CeO2+9%MgO的钛酸铝复相陶瓷经 1450 ℃烧结2 h就可以获得较好的综合性能.     

Kinetics and Mechanisms for Nitridation of Floating Aluminum Powder

Aluminum powder entrained by ammonia-containing nitrogen gas was reacted at various temperatures and time to form aluminum nitride powder. The kinetics of nitride formation were determined by a quantitative X-ray analysis and compared with those determined by a nitrogen analysis of the product. The conversion to aluminum nitride increased with the reaction time following a sigmoidal rate law. The reaction time for full conversion decreased as the temperature increased in the range 1050°–1300°C. The reaction rate constant at a given temperature was evaluated using the Avrami equation. The activation energy for the reaction was 1054 ± 91 kJ/mol in the temperature range of 1050°–1200°C, and decreased to 322 ± 70 kJ/mol above 1200°C. Comparative analysis of powders formed below and above 1200°C suggested strongly that the rate-controlling step changed from chemical reaction to mass transport above 1200°C.     

氧化反应结合SiC基陶瓷的制备与性能

本文采用反应结合制备方法,通过对坯体进行预氧化使ＳｉＣ颗凿表面氧化形成ＳｉＯ２,而后在烧成中与添中剂ＡＩ２Ｏ３－Ｙ２Ｏ３反应,使坯体气化率减少,制备了多孔ＳｉＣ基陶瓷。文章探讨了坯体中ＳｉＣ的氧化特征、反应结合过程和相变化以及它们对烧结体性能的影响。     

Properties of Different Aluminum Oxide Powders and Ceramics Made from Them

Refractories and Industrial Ceramics - Different aluminum oxide powders CL 370, CT 1200 SG and CT 3000 SG of Almatis GmbH (Germany) were studied to determine their service ability for obtaining...     

The Reaction-Bonded Aluminum Oxide Process: I, The Effect of Attrition Milling on the Solid-State Oxidation of Aluminum Powder

The effect of attrition milling on the solid-state oxidation of aluminum powder is important for the reaction-bonded aluminum oxide process. Attrition milling increased the surface area to 14.4 and 20.2 m²/g versus 1.2 m²/g for unmilled powder and smeared the Al particles, and the surface was hydrolyzed to form bayerite and boehmite. Upon heating the hydroxides decompose to form an 11–13 nm thick amorphous plus γ-Al₂O₃ layer which subsequently retards oxidation kinetics. The oxidation per unit area decreases for the higher surface area powders at temperatures below the critical temperature but the total oxidation of the milled powder is ∼70% versus ∼9% for the as-received powder because of the higher surface area. The critical temperature depends on Al particle surface characteristics and is defined as the transition temperature above which the initial rate of oxidation is linear, not parabolic. Above the critical temperature the oxidation per unit area decreases significantly. In addition, linear oxidation occurs faster than parabolic oxidation and thus the initial fast oxidation kinetics (i.e., linear) can cause thermal runaway during oxidation. Therefore, oxidation below the critical temperature is essential to maximize solid-state oxidation and to prevent thermal runaway. The critical temperatures for the as-received (1.24 m²/g), the 6 h (14.4 m²/g), and 8 h (20.2 m²/g) attrition-milled Al powders were 500°, 475°, and 500°C, respectively. A model for oxidation during the parabolic and linear oxidation stages is presented.     

The Reaction-Bonded Aluminum Oxide (RBAO) Process: II, The Solid-State Oxidation of RBAO Compacts

The oxidation kinetics and the fraction of aluminum that is oxidized via solid–gas reaction in reaction-bonded aluminum oxide (RBAO) compacts are shown to be strongly dependent on the oxidation temperature and the characteristics (size and green density) of the RBAO compact. Based on the Biot number, the oxidation process of RBAO compacts is controlled by convective heat transfer. Low heat transfer from the surface of the compact results in too-rapid oxidation, thermal gradients, and core–shell oxidation of the compacts. Uniform oxidation of RBAO compacts is possible by oxidizing at low temperatures (400°–470°C), where slow surface reaction of the aluminum particles controls the oxidation of the compact. A grain model is presented to cover both linear and nonlinear oxidation regimes during the oxidation of a RBAO compact, and this model predicts the experimental results when surface reaction of the aluminum particles is the rate-controlling mechanism and oxidation of the compact occurs uniformly.